1. Field of the Invention
The present invention generally relates to a method and apparatus for reducing pollutants by improving the conversion efficiency of a catalytic converter for treating exhaust gases produced from the combustion of fuels, and more particularly, to such a method and apparatus wherein the reduction in pollutants is achieved by the use of ozone.
2. Background
Internal combustion engines, which operate by the controlled combustion of hydrocarbon fuels, produce exhaust gases containing complete combustion products of CO.sub.2 and H.sub.2 O and also pollutants from incomplete combustion, such as CO, which is a direct poison to human life as well as unburnt hydrocarbons (HC). Further, due to the very high temperatures produced by the burning of the hydrocarbon fuels, thermal fixation of nitrogen in the air results in the detrimental formation of NO.sub.x.
The quantity of pollutants varies with many operating conditions of the engine but is influenced predominantly by the air-to-fuel ratio in the combustion cylinder such that conditions conducive to reducing carbon monoxide and unburnt hydrocarbons (a fuel mixture just lean of stoichiometric and high combustion temperatures) cause an increased formation of NO.sub.x and conditions conducive to reducing the formation of NO.sub.x (rich fuel mixture and low combustion temperatures) cause an increase in carbon monoxide and unburnt hydrocarbons in the exhaust gases of the engine. As a result, within the region of stable operation of the internal combustion engine, a significant amount of CO, HC and NO.sub.x is emitted into the atmosphere.
Although the presence of pollutants in the exhaust gases of internal combustion engines has been recognized since 1901, the need to control internal combustion engine emissions in the U.S. came with the passage of the Clean Air Act in 1970. Engine manufacturers have explored a wide variety of technologies to meet the requirements of the Clean Air Act. Catalysis has proven to be the most effective passive system.
Auto emission catalytic converters are typically located at the underbody of the automobile and are situated in the exhaust stream from the engine, just before the muffler, which is an extremely hostile environment due to the extremes of temperature as well as the structural and vibrational loads encountered under driving conditions.
Nearly all auto emission catalytic converters are housed in honeycomb monolithic structures that are generally made of cordierite, a low-thermal-expansion ceramic with excellent strength and crack resistance under thermal shock. The honeycomb construction provides a relatively low pressure drop and a high geometric surface area that enhances the mass-transfer-controlled reactions.
An adherent washcoat, generally made of stabilized gamma alumina into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. The active catalyst contains about 0.1 to 0.15% precious metals, primarily platinum (Pt), palladium (Pd) or rhodium (Rh). The honeycomb is set in a steel container and protected from vibration by a resilient matting.
The first generation of catalytic converters, from 1976 to 1979 focused solely on the oxidation of CO and HC. NO.sub.x was decreased by engine modification and operating conditions and not addressed by use of catalysis. In contradistinction, the second generation of catalytic converters, from 1979 to 1986, not only oxidized CO and HC, but also reduced NO.sub.x.
Because NO.sub.x reduction is most effective in the absence of O.sub.2, while the abatement of CO and HC requires O.sub.2, the construction of an effective means of reducing these emissions requires that the engine be operated close to the stoichiometric air-to-fuel ratio, because, only under these conditions can all three pollutants be converted simultaneously. The use of an oxygen sensor, which is positioned before the catalyst, makes it possible to maintain the air-to-fuel ratio within the narrow window required so that three-way catalysis (TWC) is possible.
Because the exhaust oscillates from slightly rich to slightly lean as a result of system time lag in adjusting the ratio, an oxygen storage medium is added to the washcoat that adsorbs (stores) oxygen during the lean part of the cycle and releases it to react with excess CO and HC during the rich portion. CeO.sub.2 is most frequently used for this purpose due to its desirable reduction-oxidation response.
TWC technology for simultaneously converting all three pollutants comprises the use of precious metals Pt and Rh (Pt-to-Rh ratio of 5-to-1), with Rh being most responsible for the reduction of NO.sub.x, although it also contributes to CO oxidation along with Pt. The basic operation of the catalyst is to oxidize CO and HC to CO.sub.2 and H.sub.2 O and reduce NO/NO.sub.2 to N.sub.2.
Third generation converters, i.e., those from 1986 to 1992, evolved as auto operating strategies shifted to include greater fuel economy at higher operating speeds, by shutting off fuel during deceleration, which exposed TWC's to higher temperatures and a highly oxidizing atmosphere, causing Rh to react with the gamma alumina of the washcoat and form an inactive aluminate. Properly designed catalysts recover their activity to a great extent under rich fuel mixtures, however, Rh may also react with CeO.sub.2 at high temperature, reducing the activity of both. This drawback is overcome by segregating the two in washcoats with multiple layers or by stabilizing the ceria with oxides of Zr, Ba and La.
In fourth generation TWC's, which began to appear in 1995, less expensive Pd was substituted for or used in combination with Pt and Rh. While the operation of fourth generation TWC's is consistent with prior catalytic converters, there is still a significant amount of pollutants emitted into the atmosphere by the catalytically treated exhaust gases of internal combustion engines.
The recent passage of the 1990 amendment to the Clean Air Act requires further significant reductions in the amount of pollutants being released into the atmosphere by internal combustion engines. In order to comply with these requirements, restrictions on the use of automobiles and trucks have been proposed, such as, employer compelled car pooling, HOV lanes, increased use of mass transit as well as rail lines and similar actions limiting automobile and truck utilization at considerable cost and inconvenience.
An alternative to diminished automobile and truck usage is decreasing emissions by increasing the efficiency of the internal combustion engine. This approach presents a considerable challenge because studies show that 70% of automobile originated pollution is contributed by only 30% of the vehicles on the road, these vehicles typically being older models having inefficient engines that inherently produce a lot of pollution. Moreover, these older vehicles oftentimes have difficulty passing required emission tests, resulting in an increased cost to the owner for tune-ups or new catalytic converters, such maintenance procedures sometimes failing to adequately cure the problem so that the vehicle still cannot pass the required emissions test.
In addition, while considerable gains have been made in recent years to reduce the amount of pollutants in the exhaust gases of the internal combustion engine of vehicles such as automobiles and trucks, it is difficult and expensive to further reduce the amount of pollutants in the exhaust gases of the internal combustion by increasing the efficiency of engines now being designed for use in new vehicles, even though exhaust emissions of automobiles and trucks currently being manufactured do not meet proposed Environmental Protection Agency standards.
In lieu of decreasing exhaust emissions by increasing the efficiency of the internal combustion engine or decreasing the use of automobiles, a further alternative would be to increase the efficiency of the catalytic converter, however, only minimal success has been achieved.
In this regard, the conversion efficiency of a catalytic converter is measured by the ratio of the rate of mass removal in the catalytic converter of the particular constituent of interest to the mass flow rate of that constituent into the catalytic converter. The conversion efficiency of a catalytic converter is a function of many parameters including temperature, stoichiometry, the presence of any catalysts' poisons (such as lead, sulfur and carbon) the type of catalyst and the residence time of the exhaust gases in the catalytic converter. At start-up the conversion efficiency of a catalytic converter is low.